1. Field of the Invention
The present invention relates to circularly polarized antenna apparatuses suitable for a variety of wireless communication systems using two different frequency bands.
2. Description of the Related Art
Wireless communication systems using 2.4 and 5.2 GHz wireless local area networks (LANs) are used these days to transmit various types of audiovisual (AV) data including television (TV) pictures.
To successfully receive such AV data, a communication error rate in wireless transmission needs to be minimized. Since communication errors become apparent on the screen in the forms of block noises and stops of moving images, the requirement for minimizing the communication errors in transmitting AV data is higher than that in general wireless data transmission such as transmission over the Internet.
In wireless transmission of AV data, therefore, circularly polarized antennas are used to minimize the communication error rate. The reasons for use include circularly polarized waves being resistant to multipath interference and being not dependent on the antenna orientation.
First, the reason for the circularly polarized waves being resistant to multipath interference will be described.
When a single type of linearly polarized waves are used, the waves cancel each other out due to reflection from obstructions, resulting in a plurality of dead spots.
On the other hand, when the circularly polarized waves are used and reflected by obstructions, the traveling direction instead of the rotational direction is reversed. In other words, with respect to the traveling direction, the rotational direction of the circularly polarized waves is reversed, and thus, multipath interference caused by reflected waves is minimized. Therefore, the circularly polarized waves are more resistant to multipath interference compared to the linearly polarized waves.
Next, the reason for the circularly polarized waves not being dependent on the antenna orientation will be described.
When a single type of linearly polarized waves are used and antennas of a transmitter and receiver are different in orientation, the communication error rate increases due to a reduced gain.
On the other hand, when the circularly polarized waves are used, the communication error rate remains substantially unchanged regardless of the changes in antenna orientation, since a gain remains unchanged even though antennas of a transmitter and receiver rotate about the traveling direction.
Examples of known circularly polarized antennas include single band antennas designed for 2.4 GHz and 5.2 GHz, but no circularly polarized antenna for dual band exists.
Therefore, two independent circularly polarized antennas are required, as shown in FIG. 3, for use in dual-band wireless communication units.
The following describes the circularly polarized antennas in FIG. 3.
FIG. 3 illustrates a circularly polarized antenna A 301 for a first frequency band and a circularly polarized antenna B 302 for a second frequency band.
A flat antenna 301b is disposed on a dielectric plate 301a to form the circularly polarized antenna A 301, which is disposed on a surface of a substrate 303. The flat antenna 301b is connected via a wiring pattern on the surface of the substrate 303 to a coaxial cable 305.
Similarly, a flat antenna 302b is disposed on a dielectric plate 302a to form the circularly polarized antenna B 302, which is disposed on a surface of a substrate 304. The flat antenna 302b is connected via a wiring pattern on the surface of the substrate 304 to a coaxial cable 306.
The circularly polarized antennas A 301 and B 302, which are different in size and frequency band characteristics, have hemispherical radiation patterns and are disposed such that their waves are opposite in direction.
Furthermore, to provide a diversity antenna apparatus using the above-described circularly polarized antennas, at least two antennas are required for each of two frequency bands, that is, a total of four or more independent antennas need to be mounted on the dual-band wireless communication unit (see, for example, Japanese Unexamined Patent Application Publication No. 2002-43994).
In a known circularly polarized antenna, as described above, a combination of two single-band antennas is required for providing a dual-band antenna apparatus, and further, a combination of four single-band antennas is required for providing a diversity antenna apparatus. This interferes with the reduction in size of the wireless communication unit.
Moreover, when coaxial cables connected to antennas are mounted on the wireless communication unit, the coaxial cables function as antennas and might emit and detect noises to and from the wireless communication unit and other peripheral devices. The noises caused by the coaxial cables seriously affect the dual-band wireless communication unit, which requires four coaxial cables.